About PD passive

Does anyone else share the same though about the Phantom Dancer passive shield being just incredibly overpowered? As Riven, with Death's dance and Duskblade, ulting a 20% health vayne ( Riven's R scales off enemy's percent missing health ), it did literally no damage. I understand it being an item/passive to counter assassins, give you some time to survive, but really? It just denied probably about 650-700 damage completely. How is this balanced?
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